Spin-On Filters and EDM Filters
Prior art discloses various valves to direct the flow of dirty fluid inside a filter in a variety of applications including hydraulic, fuel, and engine lubrication systems. In automotive spin-on oil filters, for example, pressure relief valves are designed to provide circulating engine oil a by-pass route when the filter is restricted due to dirt accumulation or high oil viscosity because of a cold start in a winter climate. Some examples of known designs can be found in U.S. Pat. No. 4,127,484 (Walulik et al.), U.S. Pat. No. 5,584,987 (Mules), U.S. Pat. No. 5,690,816 (Covington), and U.S. Pat. No. 6,096,199 (Covington). These filters have the now familiar design of a casing enclosing a cylinder of filter media therein. Other previous designs include U.S. Pat. No. 2,998,138 (Mould et. al.) and U.S. Pat. No. 3,370,708 (Hultgren), these each incorporating a pair of concentric, cylindrical elements of filter media, this being a characteristic they share with the invention.
Various filters have been designed for the filtration requirements of EDMs (Electrical Discharge Machines). These machine tools require the filtration of large volumes of a high dielectric fluid, either dielectric oil or deionzed H20. The EDM filters discussed here are one component in a continuous loop EDM filtration system which includes a work table tank 31, a pump 33 to conduct the dirty work fluid from there to the filter 35, a clean side tank 37 or container to capture the clean filtrate, and a second pump 39 to bring the filtrate back to the work table area in order to repeat the cycle. The flow path of the EDM fluid in a typical EDM is shown schematically in FIG. 1.
Unlike the valves in spin-on oil filters, in an EDM filter valve there is no requirement for temperature sensitivity or an on-off feature. These circumstances present an opportunity for an EDM filter with an improved, more simple valving arrangement.
The invention does not include a spring in its design, such as in the spring-biased valves described in U.S. Pat. No. 4,557,829 (Fields) and U.S. Pat. No. 5,733,443 (Stamey et al.) or in the aforementioned U.S. Pat. No. 5,584,987 (Mules) and U.S. Pat. No. 3,370,708 (Hultgren). A valve utilizing a spring would be inherently more complicated than the invention. A spring in an EDM filter would require that its cross-section and materials of composition be considered. Changes in friction in the facing surface as the spring loads up need be engineered. Similarly, relief valve assemblies can be unduly complicated, require close assembly tolerances, and are not easily installed on filter assembly lines featuring rapidity without precise installation equipment.
The sludgy inside of a working EDM filter would be a challenging environment for any mechanical valving device. It is a commonplace of the EDM filter industry that a EDM filter's service life varies depending on the type of material being cut. The effect(s) of various kinds of dirt deriving from the commonly EDMed metals (tool steel, titanium, aluminum, copper, etc.) impinging on the spring's performance would have to be taken into consideration. In addition, an EDM filter sees internal pressure load changes as the EDM cycles or when the machine is turned off. The invention meets these challenges with a simple, elegant valving arrangement involving no moving parts. It is an object of the invention to provide an improved two stage liquid filter design wherein the disadvantages of prior filter constructions are obviated.
EDM Filters: Inside-Out Plumbing Arrangement
One EDM filter engineering approach employs a filter where the dirty fluid is routed inside-out, that is, into the interior of the filter by means of a conduit such as an inlet pipe with apertures that runs through the middle of the filter, this inlet pipe being part of the original equipment on the EDM, a sealed cylinder of filter medium through which the filtrate passes under pressure, and an external cylindrical cage to support the filter medium cylinder. A pleated filter medium cylinder 41 and an inlet standpipe with apertures 43 only are illustrated in FIG. 2. Note the inside-out flow path.
Because of the large amounts of metal sludge generated by late model EDMs, these inside-out filters tend to be large, often the size of an office waste paper basket, in order to collect as much sludge as possible between change-outs. As compared to the more common outside-in style filter, the inside-out filter offers the advantage of encapsulating almost all the metal sludge inside the filter at change-out.
The inside-out approach in an EDM filter offers the further advantage of a large internal volume that can be utilized for filtration purposes. It is the object of the invention to utilize this space more economically than heretofore.
An inner cylinder of pleated filter paper is sometimes included in these inside-out filters. Thus there are two concentric cylinders of pleated filter media. In this dual cylinder approach, the filtered fluid passes through the inner and outer cylinder simultaneously in FIG. 3. An increase of 40 percent in available overall filter surface area is typically obtained with a corresponding increase in the filter's useful life.
Re-plumbing an EDM's filtration system originally equipped with the single cylinder filter set-up to accept a dual cylinder filter is possible. However, the disadvantages of the dual cylinder approach, as currently designed, are the increased costs occurred in three areas:
1) the additional fixtures: a male plug 45 or nozzle on the filter itself, a female coupling device 47 on the hose, a hose clamp and hose 49, shown in FIG. 3.
2) a new filter tank or housing that accommodates the operator's need to access the coupling.
3) the additional facility space required for the plumbing fixtures and the change-out operation just mentioned
The dual cylinder approach would become more attractive if it could be utilized in the existing EDM filter tank arrangements without radical re-plumbing. Our market research has shown the most EDM owners, faced with the effort and expense of re-plumbing their EDM, choose to not take advantage of the extra filter media that could be incorporated into an EDM filter's interior. Therefore, what is needed is a user-friendly, dual cylinder inside-out filter that utilizes the existing tank and plumbing fixtures. Preferably, this filter would be economical to produce and lend itself to high volume manufacturing operations.